The invention relates generally to the field of CT imaging and more specifically to a distributed source configuration for the imaging of dynamic internal tissues. In particular, the invention relates to an interpolation-based reconstruction technique for performing full field of view imaging of dynamic internal tissues, using the source configurations.
Computed tomography (CT) imaging systems measure the attenuation of X-ray beams passed through a patient from numerous angular positions about the patient. Based upon these measurements, a computer is able to reconstruct images of the linear attenuation coefficient of the portions of a patient's body responsible for the radiation attenuation. As will be appreciated by those skilled in the art, these images are based upon separate examination of a series of angularly displaced images of the transmitted X-ray beam intensity. A CT system processes X-ray intensity data to generate two-dimensional (2D) maps of the line integral of linear attenuation coefficients of the scanned object at multiple view angle positions about the object, denoted as projection data. These data are then reconstructed to produce one or more images, which are typically displayed on a monitor, and may be printed or reproduced on film. A virtual three-dimensional (3D) image may also be produced by a CT examination.
CT scanners operate by projecting fan-shaped or cone-shaped X-ray beams from an X-ray source. The X-ray beams may be collimated to control the shape and spread of the beams. The X-ray beams are attenuated as they pass through the object to be imaged, such as a patient. The attenuated beams are detected by a set of detector elements. Each detector element produces a signal affected by the attenuation of the X-ray beams, and the data are processed to produce signals that represent the line integrals of the attenuation coefficients of the object along the X-ray paths. These signals are typically called “projection data” or just “projections”. By using known reconstruction techniques, such as filtered backprojection, useful images may be formulated from the projection data. The images may in turn be associated to form a volume rendering of a region of interest. In a medical context, pathologies or other structures of interest may then be located or identified from the reconstructed images or a rendered volume.
CT imaging techniques, however, may present certain challenges when imaging dynamic internal tissues, such as the heart. For example, in cardiac imaging, the motion of the heart causes inconsistencies in the projection data, which, after reconstruction, may result in various motion-related image artifacts such as blurring, streaking, or discontinuities. To reduce the occurrence of motion-related image artifacts, various techniques may be employed to improve the temporal resolution of the imaging system, thereby reducing the effects of the moving tissue. Temporal resolution may generally be improved by decreasing the rotation time of the CT gantry. In this way, the amount of motion that occurs within the temporal window associated with the acquisition of a projection data set is minimized.
Temporal resolution may be further improved by the choice of reconstruction algorithm. For example, segment reconstruction algorithms, such as half-scan reconstruction algorithms, may be employed in the reconstruction process. The segment reconstruction algorithms typically reconstruct images using projection data collected over an angular range of 180° plus the fan angle (β) of the X-ray beam. Because the acquisition of projection data during gantry rotation of 180°+β requires less time when compared to acquisition occurring during 360° of gantry rotation, the temporal resolution in the reconstructed images is improved.
Multi-sector reconstruction techniques may also improve the temporal resolution of the reconstructed images by using projection data acquired during multiple rotations of the gantry by a multi-slice detector array. The projection data set used for reconstruction are composed of two or more sectors of projection data that are acquired during different cardiac cycles. The sectors comprise the data acquired during a short span of the gantry rotation, typically less than half of a rotation. The sectors, therefore, have good temporal resolution if acquired by a rapidly rotating gantry, thereby providing good effective temporal resolution for the aggregate projection data set used in reconstruction.
Using the techniques discussed above, third and fourth generation CT systems are capable of temporal resolutions of approximately 250 ms using segment reconstruction techniques. Fifth generation CT systems, utilizing a stationary detector ring and an electron gun which sweeps an electron beam along a stationary target ring to generate x-rays, are capable of achieving a temporal resolution of approximately 50 ms or less. A temporal resolution of approximately 20 ms, however, is desirable in order to “freeze” cardiac motion, thereby minimizing motion-related artifacts in the reconstructed images. While such fifth generation systems could be made to scan faster, they suffer from a non co-planar detector and source configuration. The fact that the source and detector do not rotate means that at some subset of angles in the scan, the detector is occluded by the source (or visa versa). As a result, such systems tend to collect incomplete data, and suffer from image artifacts as a result. For third generation CT systems, improving temporal resolution in addition to the above techniques has typically focused on further increasing the rotational speed of the gantry.
However, as the rotational speed of the gantry increases, the centripetal force required for gantry components also increases. The increasing centripetal force and the tolerances of the gantry components may comprise, therefore, a mechanical limitation to increases in gantry angular velocity. Furthermore, to obtain consistent image quality in terms of signal-to-noise ratio, a high integrated X-ray flux should be delivered to the imaged object or patient during the scan interval. However, achieving a high integrated X-ray flux for faster rotation of the gantry requires increased instantaneous X-ray flux and places increased demand on the X-ray tube, particularly in regard to tube output, and on the components that cool the X-ray tube. Both mechanical and X-ray flux considerations, therefore, are obstacles to increasing the gantry rotation speed sufficiently to achieve a temporal resolution of 20 ms or better in CT reconstructions. A technique for achieving a high temporal resolution without increasing gantry rotation speed is therefore desirable.
Furthermore, it is also desirable to develop CT scanners with high spatial and temporal resolution, good image quality, and good coverage along the z-axis, i.e., the longitudinal axis of the CT scanner. However, existing systems typically acquire projection data for a limited extent of the patient or object being scanned. Therefore, it may be desirable to increase the coverage of the detector in one or more dimensions to facilitate measurement of projection data from the entire portion of the object or subject being scanned. For example, longitudinal axis coverage of the detector may be improved by increasing the number of rows of detector elements in the detector. This approach has lead to the development of CT systems with larger detectors. Larger detectors, however, may be undesirable for a variety of reasons. For instance, as one might expect, larger detectors and associated acquisition electronics are both more costly and more difficult to produce. In addition, the mechanical subsystem responsible for supporting and/or rotating a larger detector may also need to be larger and more complex and/or may be subject to greater mechanical stress. Furthermore, large detectors are associated with increased cone angles, i.e., the angle between the source and the detector periphery in the longitudinal direction. The increased cone angle between the source and detector periphery is in turn associated with increased cone-beam artifacts in the reconstructed images depending on the choice of data acquisition protocol and reconstruction algorithm. When the cone angle increases beyond a certain limit, the degradation of the image quality may become severe for axial, or step-and-shoot scanning. For the foregoing reasons, increasing the scan coverage by simply increasing the detector coverage, i.e., size of the detector, is not a sufficient or complete solution.
A technique for achieving high spatial resolution and high temporal resolution, good image quality, and good coverage using a standard or smaller detector is therefore desirable. In addition, it is also desirable to develop a technique for achieving high temporal resolution without substantially increasing the rotation speed of the gantry.